The invention in general relates to methods and apparatus for detecting the velocity of blood flowing in a vessel. More particularly the invention relates to methods and apparatus for producing an audio signal representative of the velocity of blood in order to provide an instrument that can be used to routinely measure the velocity of blood flow in vessels, such as in the retina of the eye, and which can be used by any medical person.
The ability to measure the velocity of blood flowing in a single blood vessel or in a capillary bed is very useful for medical purposes. The restriction of blood circulation in various parts of the body has long been associated with disease and degeneration of both particular organs and the body as a whole. In particular it has become an accepted fact that impairment of blood flow in the tissues of the ocular fundus, or the retina of the eye, is associated with a large number of diseases that can lead to grave visual disorders. Blood flow measurements have also been used in the case of massive wounds to differentiate between tissues which have been killed, and which therefore must be removed to prevent gangrene, and tissues which are still living, and thus should not be removed.
Up to now information on blood flow in the body has been obtained by injecting dyes into the circulatory system and measuring the speed of the spread of the coloration. For example, retinal blood flow has been measured by fluorescein high speed cinematography. In this technique, fluorescein dye is injected into the circulatory system and the passage of the dye bolus in the fundus of the eye is recorded with a movie camera. The velocity of the blood is determined by measuring the distance traveled by the dye profile between successive frames of the film. This method is not adequate for routine clinical work, however, because it requires manipulating a catheter into the carotid artery. The information developed is also not immediately available because the film must be developed, and this usually requires a wait of at least several days before the film is returned from the film processing laboratory.
Recently a new technique for measuring the blood velocity in the retina of the eye has been demonstrated by this inventor. (See for example, Laser Doppler Measurement of Blood Flow In The Fundus Of The Human Eye, by C. E. Riva and G. T. Feke in Proceedings of the 1976 Electro-Optical Systems Design/International Laser Conference, pp. 142-147). This technique makes use of laser Doppler velocimetry (LDV). In LDV laser light is scattered from moving particles and the well-known Doppler effect leads to a frequency shift f between the frequency of the light scattered from the moving particle and the incident light frequency. The frequency shift f is related to the velocity V, the light wavelength .lambda. (in vacuo), and the scattering angle .theta. by the formula: EQU f = V/.lambda. sin .theta.
In the new technique light is impinged on blood flowing in a blood vessel and the reflected light is detected by a photodetector. The light impinged on the vessel wall may be specularly reflected; however the term "reflected" when referring to the blood cells means non-specularly reflected, or scattered. The non-specularly reflected light is Doppler frequency shifted. The frequency shift given by the above formula then appears explicitly in the photocurrent output of the detector as a result of the inherent optical mixing process on the photocathode. Since normally in a blood vessel the blood cells flow at velocities varying over a range of velocities the photodetector output is a Doppler shift frequency spectrum (DSFS) which contains a range of frequency shifts corresponding to the range of particle velocities. In the technique demonstrated the frequency spectrum is plotted on a chart recorder as a function of the number of red blood cells which give rise to the Doppler shift of frequency f. The chart is then interpreted to give the blood velocity profile in the blood vessel and to diagnose any disorders associated with the indicated blood flow, or lack of blood flow.
The above described technique can be used to measure the blood flow in either an isolated vessel, or in a capillary bed. When the measurement is made on an isolated vessel light is reflected both from the wall of the vessel and from the red blood cells flowing in the vessel. Since the vessel wall is not moving the light reflected from it is not shifted in frequency. This light of unshifted frequency reflected from the wall of the vessel is incident on the photodetector along with the light of shifted frequency from the moving blood cells. The light of unshifted frequency acts as a reference beam the frequency of which is substracted out from each of the shifted frequencies by the optical mixing process in the photodetector, so that the output of the photodetector gives the frequency shifts directly. This type of optical mixing is called heterodyne mixing.
If the blood velocity is being measured in a capillary bed the reflectance from the tiny capillary walls is not sufficient to give heterodyne mixing. Thus the photodetector substracts one shifted frequency from another to give an output equivalent to a convolution of the heterodyne spectrum with itself. The resulting spectrum, called a homodyne spectrum, still contains all the information necessary to obtain the spectrum of blood velocities and thus can also be used as a diagnostic tool. The blood velocities in the capillary bed of the optic nerve in the retina of the eye are particularly important in diagnosing glaucoma.
Measurements of DSFS from red blood cells moving in tissues other than those in the eye have also been made. See for example M. D. Stern, "In Vivo Evaluation Of Microcirculation By Coherent Light Scattering", Nature, 1975, p. 56, Vol. 254.
The above described LDV techniques for measuring blood velocity have many advantages over the dye techniques. They are non-invasive and thus there is less chance that the measurement itself will affect the circulation system being measured. The laser intensities used are well below the intensities which might damage the tissue being preserved. In addition, use of a catheter, which requires highly trained experts, is not required. However the output of the above described systems is still in a form that is difficult to use in routine office or hospital tests because it requires bulky equipment and substantial skill and analysis.